Fundamentals of small-part turning

Turning small parts is nothing new for many machine shops. Industries like automotive, aerospace, electronics, firearms, and medical are pushing the envelope when it comes to small-part production. Typically, any application requiring a workpiece smaller than 1 in. dia. is considered small-part turning.

Micromachining, on the other hand, is classified by part diameters that are too small to be fixtured using a standard collet. In some instances, this could mean parts that are 0.1 in. to 0.02 in. dia. all the way down to the size of a human hair, depending on the equipment.

For shops looking to take on small-part turning operations, there are a number of things that can be done to ensure success.

Tooling

Turning small components requires a different style of tooling than large-part turning.

"Sharp molded and ground cutting tool inserts provide less tool pressure and freer cutting," said Steve Easterday, Swiss applications engineer, Kyocera Precision Tools, Hendersonville, N.C. "Physical vapour deposition (PVD)-coated inserts provide excellent tool life and a sharper edge."

With small components, tool pressure is a primary concern. Very small parts can easily deform during turning operations.

"Because of this, the process needs to be planned to minimize tool pressure using sharp, positive tools," said Edwin Tonne, training and technical specialist, Horn USA Inc., Franklin, Tenn. "It’s important to look for well-defined cutting edges and a positive top rake. Fully sintered cutting tools can be problematic because they’re just not sharp enough to minimize the pressure. Look for finely ground cutting edges with plenty of clearance. A sharp wedge angle will help prevent deformation in the part. Form tools are a good choice because they can simplify the process."

Chip control and evacuation can be cumbersome in small-part turning. For the most part, the biggest tool possible is the better option, but it’s important to remember that there must be enough room, especially in boring operations, for chip evacuation. High-pressure coolant can be a huge help with this.

"The No. 1 thing to consider is chip control," said Easterday. "Having the correct chipbreaker is key. The chip needs to be directed away from the workpiece."

Looking for tooling that is purposely designed for small-part turning or microturning is a good place to start. These tools tend to have tighter tolerances and are more finely ground.

"With small tools, a high-quality modular system will allow you to keep a close eye on the tool wear," said Tonne. "If you have a feature that’s only 0.001 in. deep, and you lose the radius of your tool, you can have a problem on your dimensional tolerance, so keeping a close eye on the tool is very important."

Operators need to be mindful of the cutting edge at all times. It needs to be sharp, and damage can occur very easily from general wear, handling, or even indexing.

With very tight tolerances, it can be challenging to get the tool back to size after indexing. Monitoring and controlling the size after indexing the cutting tool is a must for small-part turning.

Another thing to consider is keeping plenty of extra hardware for the holder around because typically the screws holding the insert tend to be very tiny.

"The screw could be an M2.5 or smaller, and in some cases, they can be easily lost to the bottom of the machine or conveyor system," said Tonne. "The other thing I would recommend is using torque-limiting drivers at all times. If you overtorque an M2.5 screw, you’re going to strip it, and that will add a lot of extra time getting that screw out and changing the insert."

Holder alignment and pocket quality are important when dealing with microtolerances and should be checked frequently. Although these operations generally use low tool pressure and there isn’t significant stress in the pocket, over time wear can occur, so it’s important to monitor it.

"When working with anything less than 0.250 in. dia., it becomes difficult to reach the correct speed for normal steel material," said Tonne. "And with RPM limitations on the machine, sharp cutting edges and very lubricious coatings are helpful. When it comes to depth to cut, typically the machine will perform a single finish pass. This needs to be balanced with tool pressure. First and foremost, keep your feeds very modest. Form tools and special tooling can help claw back some of the time."

Because of the low cutting speed required, built-up edge can be a problem. On larger parts burrs can be removed easily with the secondary processes, but it’s more difficult with small parts.

"Common mistakes can include incorrect cutting conditions like incorrect surface footage, too high feed rate, and too heavy depth of cut, which can result in incorrect tolerances that include part bending and poor finish," said Easterday.

Machine

Depending on the size of the parts and specifications needed, small-part turning can be done on a conventional CNC.

"Many of today’s new machines come equipped to handle smaller parts," said Bill Mara, president, EMEC Machine Tools Inc., Mississauga, Ont. "One of the biggest challenges machine shops face when dealing with small-part turning is the machine’s capability to hold the parts effectively without damaging or distorting them, but that’s not really a problem anymore. For shops looking to get into parts that are smaller than 0.250 in. dia., a Swiss-style lathe is the way to go."

A conventional CNC lathe is good for general-purpose turning but usually has spindle speeds of only about 3,000 to 5,000 RPM, whereas a Swiss-style lathe can be in the 10,000- to 15,000-RPM spindle range. That higher RPM enables the machine to generate the surface speed needed to cut small-diameter parts efficiently.

"For shops looking at turning long and thin parts, a conventional CNC lathe has its limitations and probably wouldn’t perform as needed," said Mara. "A Swiss-style machine offers the rigidity in the setup required to turn very small or long parts by doing the cutting as close as possible to the spindle nose and by feeding the material through a stationary tool as opposed to the tool moving across the part as on a conventional lathe. Choosing a Swiss-style machine is definitely the best option, if possible, for turning long, thin parts."

On a conventional lathe you may be able to turn a length-to-diameter (L:D) ratio of about 10:1, and that is only if the machine has a steady rest or similar supporting device. On a Swiss-style machine, the L:D can be as much as 20:1, making it possible to produce very difficult parts.

"Shops have been cutting small parts forever and have found innovative ways to do it effectively," said Mara. "It’s important to remember that a machine needs to have a rigid setup, regardless of whether it’s a conventional or Swiss machine, and it is vitally important that the correct tooling is used."

The enemy of precision machining is vibration and heat. Rather than heavy, deep cuts, small, higher-speed cuts can keep the workpiece as cool as possible, reducing heat deformation in the part. Any sort of deformation can make it difficult to maintain proper tolerances. According to Mara, Swiss machines usually use high pressure up to 2,000 PSI with water-based coolant to help keep the heat out of the part.

"If their machine is equipped with HP coolant pumps, internal coolant through tooling is a must," added Easterday. "Make sure that the workpiece can be done on the machine you have. In many cases when you are dealing with long workpieces, adjustments need to be made. Extended guide nose bushings or the capability to pinch-turn can be helpful."

With a long part it’s always a good idea, especially with a small diameter, to try to grab it with the secondary spindle. While this may add a little bit of time, gripping and not letting the part hang out adds the stability needed to ensure tolerances are met.

Tonne agreed that when working with long parts, pinch turning is a great option. Pinch turning helps keep everything aligned. The external features are completed first to maintain that stability and then internal operations are performed.

"One big mistake is not checking the alignment of the machine," said Tonne. "Alignment is everything."

Measurement

Another area of importance is measurement and inspection. Small components require different inspection methods than their larger counterparts, especially because of the microtolerances and tiny features.

"You may want to think about using a vision inspection system, offline gauges, and those sorts of devices," said Tonne. "These take more time than using a caliper or bench-type measurement tool. Getting the part to the right size and back up and running after tool indexing is also critical."

Automation

Automation is slowly making its way into all aspects of the shop floor. However, when working with small parts, automation may not be the obvious next step.

"Everyone wants to automate their process," said Mara. "We can’t get skilled people, so the logical step is to automate. End-of-arm tooling is not typically as dextrous as it needs to be for small parts, and robot accuracy may not be sufficient. It’s difficult to hold very small parts with a robot, especially for loading and unloading."

Another concern with small parts is they get washed away with chips. For example, part ejectors that put small parts into the conveyor may have issues separating them from chips. A vacuum that removes the part when the chuck opens and puts it in a separator is a good option.

"Automation for small parts has historically been a challenge, but we are really starting to see some new and exciting advancements in automation," said Mara.

Associate Editor Lindsay Luminoso can be reached at lluminoso@canadianmetalworking.com.

EMEC Machine Tools Inc., www.emecmt.com

Horn USA, www.hornusa.com

Kyocera Precision Tools, www.kyoceraprecisiontools.com

Tsugami, www.tsugami.co.jp/eng